In a laser system in which a diode laser and a collimating lens is used, ambient temperature changes cause the laser, lens, and the mounts therefor to expand or contract or otherwise change the characteristics thereof. This causes a net change in focus of a laser beam on a print medium (e.g., thermal dye transfer medium, etc.) or other end surface of the laser beam. It is desirable to maintain the focus over a predetermined temperature range for normal laser printing operations. Various techniques have been developed for maintaining the focus of a laser and lens combination over a predetermined temperature range.
U.S. Pat. No. 4,641,023 (H. Ando et al.), issued on Feb. 3, 1987, discloses an optical head for an optical disc wherein a semiconductor laser is supported on a base which is secured to a hollow frame. A lens-barrel for supporting a collimator lens system is disposed in the hollow frame so that the collimator lens system faces the laser. The distance between the collimator lens system and the semiconductor laser is allegedly maintained although the disclosure incorrectly states that the distance that is maintained constant is between the front focal plane and the semiconductor laser. The problem with this arrangement is that the front focal plane of the lens changes with temperature which is not taken into account.
U.S. Pat. No. 4,720,168 (Y. Kaneko), issued on Jan. 19, 1988, discloses a laser beam collimating apparatus comprising a semiconductor laser, a collimating lens, and a support for holding the laser and the lens at a predetermined distance from each other. The material of the support is stated as having a linear expansion coefficient which can cancel out a deviation of the focal distance of the collimator lens due to a wavelength change of the semiconductor laser over temperature. More particularly, the wavelength change of the semiconductor laser induces a focal length change of the lens which is compensated for by the material chosen. However, the remaining problem is that this does not take into account how the lens itself changes with temperature.
U.S. Pat. No. 4,815,059 (M. Nakayama et al.), issued on Mar. 21, 1989, discloses a device comprising a semiconductor laser for emitting a laser beam, a coupling lens for collimating the laser beam, and a support member for supporting the laser and the coupling lens. The support member is structured so that the focal distance of the coupling lens varies in accordance with a change in wavelength of the laser beam caused by a change in ambient temperature. The problem with this device is that the design of the device is fixed because a particular lens is used which is designed specifically for the device.
U.S. Pat. No. 4,948,221 (T. Yates), issued on Aug. 14, 1990, discloses an athermalized optical head comprising a light source (e.g., a laser diode), and an optical element (e.g., a collimator lens). The light source and the optical element are mounted along a common axis, and a thermoelectric cooling element is provided in the optical head to control the temperature of the light source. The distance between the light source and the optical element is maintained constant over a predetermined temperature range to maintain a spot produced by the head in focus. A problem with this optical head is that the design tries to maintain the distance constant between the light source and the collimator lens even though changes occur in the collimator lens with changes in temperature.
U.S. Pat. No. 4,993,801 (S. Sarraf), issued on Feb. 19, 1991, discloses an optical head comprising a light source (e.g., a laser diode) and an optical element (e.g., a collimating lens). The light source and the optical element are mounted along a common optical axis, and a thermoelectric cooling element is provided in the head to control the temperature of the light source. To precisely locate the optical element relative to the light source, a housing of the optical element is mounted directly onto a barrel of the light source. A problem with this optical head (as with the Yates patent above) is that the design tries to maintain the distance constant between the light source and the collimator lens.
It is desirable to provide an athermally compensated printhead which is simple and inexpensive to manufacture, is able to compensate for various effects due to ambient temperature changes and maintain the focus of a laser beam over a predetermined temperature range, and is able to use any one of the commercially available laser diodes and lenses.